Optical temperature sensing is crucial to many applications, especially where other methods of temperature sensing are unsuited. There is a long history of optical temperature sensing by measuring thermal radiation output (i.e., pyrometry) that offers the same advantages of being a non-contact, non-electrical measurement that are common to all optical measurements. However, the accuracy of pyrometer measurements is limited by the degree to which the emissivity of the observed object is known, a value which is often uncertain or non-trivially temperature-dependent. Additionally, as any intensity-based measurement, it is perturbed by many system and environment instabilities (an important one of which is an inhomogeneous or flame atmosphere). Luminescence-based temperature sensing, when decay time rather than intensity is used for temperature indication, was developed to overcome these issues and to this purpose has been successful, although with much lower upper temperature limits than pyrometry. In some cases, luminescence temperature sensing has been combined with pyrometry to create a high precision low-temperature sensing system based on luminescence measurements with extended capability (though with less precision) at high temperatures using pyrometry.
Effective lower temperature (below 600° C.) sensing based on oxides or fluorides doped with transition metals (particularly Cr3+-based oxides like ruby, alexandrite, and emerald) have been utilized for the high luminescence intensity associated with the strong 3d to 3d absorption and emission transitions. Doping levels as small as 0.01% Cr3+ are more than sufficient to provide measurable luminescence at room temperature. In addition, due to the broadness of absorption bands in these materials, a wide variety of excitation sources can be used to excite luminescence.
Higher temperature luminescence decay sensors (some going as high as 1700° C.) have been made based on trivalent rare earth ions such as Eu3+, Dy3+, Tb3+ and Tm3+. In these ions, the luminescence is associated with energy transitions of the 4f electrons, whose shielding by the 5s and 5p electrons prevents the energy level broadening (by phonon coupling) observed in the transition metals. This leads to the advantage of suppressed non-radiative transitions, and thermal quenching is delayed to typically much higher temperatures; however, this brings the disadvantage of inherently weaker luminescence by several orders of magnitude due to weaker absorption and emission probabilities. The use of higher doping levels can compensate for the weaker luminescence to some degree; however, luminescence intensity reaches a maximum typically at a few cation percent before falling off due to concentration quenching. The low emission intensity from rare earth dopants, especially in the presence of intense background thermal radiation, has been a severe impediment for successful high temperature application.
The key difficulty encountered by the current state of the art is achieving temperature sensing capability and in maintaining temperature measurement precision to high temperatures. As temperature increases, non-radiative de-excitation processes increasingly compete with the radiative transition of the lumiphore, causing a quenching, or loss, of signal intensity at the same time that the blackbody background radiation increases. For all lumiphores, there is a temperature at which luminescent intensity is no longer adequate to reliably measure temperature. In addition, the luminescence decay time which is used to correlate to temperature decreases exponentially with temperature after the onset of quenching; this leads, on one hand, to the requisite temperature sensitivity but also results in a secondary upper limit to temperature sensing capability at which point the decay time is too short to be measured without convolution from the excitation or detection apparatus. As such, conventional non-contact, luminescence-based optical temperature sensors lack intensity and decay time properties sufficient to be used in many high temperature applications.